Mandrel lubricant for the production of seamless tubes

ABSTRACT

A mandrel lubricant for use in the production of seamless tubes. The mandrel lubricant composed of graphite, clay minerals from the smectite class, a polysaccharide and optionally a surfactant. The coating of the mandrels that have passed through a cooling bath takes place in the form of an aqueous dispersion.

This application is a Continuation of prior U.S. application Ser. No.08/012,954 filing date Feb. 3, 1993, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a mandrel lubricant for the production ofseamless tubes, especially for the coating of mandrels that have passedthrough a cooling bath after the rolling operation and are beingprepared for the next rolling operation.

2. Background Information

In modern tube rolling mills, e.g., in continuous tube trains(MPM-trains), the shaping of the seamless tubes takes place in the mainprocess step by rolling of a prefabricated ingot heated to 1200° to1300° C. over a mandrel which is mounted on a mandrel rod. After therolling operation, the mandrel or the mandrel rod is removed from thetube round and fed to a cooling bath where the mandrel or the mandrelrod is cooled down from about 150° to 350° C. to a temperature of about60° to 100° C., and in this way, is prepared for the next rollingoperation. A lubrication composition is also included in thispreparation of the mandrel or mandrel rod after the cooling bath. Thislubrication composition is essential to assure an optimal "sliding" ofthe ingot on the mandrel rod during the rolling operation and is one ofthe factors decisive for the later quality of the tube, i.e., for thecondition of the inner surface of the tube.

Generally, this lubrication composition was made withgraphite-containing oils. By burnoff of oil on contact of the mandrelwith the hot ingot, a very strong smoke development occurred that,because of the toxic components of the smoke, led to an unacceptablepollution of the surrounding area and environment. On the other hand,this uncontrolled burnoff caused by the ingot during the rollingoperation led to damage to the lubricant coating on the mandrel andpossibly to damage to the inner surfaces of the tubes.

Then, in German Patent No. 2450716, especially in Example 5, it wasproposed to use a high temperature lubricant essentially consisting ofgraphite and an alkylene polymer. This lubricant was applied immediatelyafter the rolling operation of the still uncooled mandrel, whereupon adry and partially water-resistant lubricant film formed. The mandrel sotreated was then able to be fed to the cooling bath and later, withoutrelubrication, again to the rolling process. But it has turned out thatby the mechanical stress on the mandrel in transport, by the coolingbath and by the action of the water, the lubricating film is oftendamaged which in turn has a negative effect on the quality of the tube.

Since in modern rolling trains the lubrication composition of themandrels takes place after the cooling bath, it was then attempted,corresponding to the lubrication composition with graphite-containingoils, to apply the lubricant formulation of German Patent No. 2450716 tomandrels that have already passed the cooling bath. As required by theprocess, the time between application of the lubricant to the 60° to100° C. mandrel up to the rolling operation is limited to a maximum of 5seconds.

As the present inventors' comparison tests show (see examples), even theformation of a dry lubricant film with the lubricants according toGerman Patent No. 2450716 takes at least 15 seconds. As a result, thesame effect is observed as in the treatment of the mandrel withgraphite-containing oils when the treated mandrel comes in contact withthe hot ingot. Moreover, the polymeric components are subjected topyrolysis which leads to damage to the lubricant film during the rollingoperation and, thus, finally adversely affects the quality of the tubes.

BROAD DESCRIPTION OF THE INVENTION

The main object of the invention is to provide a lubricant that does nothave the above-mentioned drawbacks and which is especially suitable toprovide mandrels that have passed through the cooling bath with aquick-drying, high-grade lubricant film. The object of the invention isachieved with the invention lubricant composed of:

(a) 60 to 95 percent by weight of a natural or synthetic graphite;

(b) 5 to 40 percent by weight of one or more clay minerals from thesmectite class;

(c) 0.2 to 2 percent by weight of a polysaccharide or one of itsderivatives; and

(d) 0 to 5 percent by weight of a non-ionic surfactant.

Other objects and advantages of the invention are set out herein or areobvious herefrom to one skilled in the art.

The objects and advantages of the invention are achieved by thelubricants and processes of the invention.

Preferably the mandrel lubricant consists of:

(a) 75 to 90 percent by weight of a natural or synthetic graphite;

(b) 5 to 25 percent by weight of one or more (i.e., at least one) clayminerals from the smectite class;

(c) 0.5 to 1 percent by weight of a polysaccharide or its derivatives;and

(d) 0 to 2 percent by weight of a non-ionic surfactant.

The invention includes the coated mandrels for the production ofseamless tubes. The mandrel having passed through the cooling bath afterthe rolling operation takes place and before the next rolling operation.The mandrel has a coating of a mandrel lubricant. The mandrel lubricantconsists of:

(a) 60 to 95 percent by weight of a member selected from the groupconsisting of natural graphite and synthetic graphite, the naturalgraphite having a crystal size L_(c) greater than 100 nm and an ashcontent of a maximum of 5 percent, and the synthetic graphite having acrystal size L_(c) greater than 100 nm and a purity of 99.9 percent orhigher;

(b) 5 to 40 percent by weight of montmorillonite with a swellingcapability in water of 3 to 50;

(c) 0.2 to 2 percent by weight of at least one member selected from thegroup consisting of polysaccharide and one of its derivatives;

(d) 0 to 5 percent by weight of non-ionic surfactant; and

(e) optionally, a biocide in an amount which is effective to protectsaid polysaccharide or one of its derivatives from bacterial attack. Theinvention also includes the seamless tubes prepared using the countedmandrels in the hot rolling operation. The invention also includes thehot rolling operation for making seamless tubes using the coatedmandrels.

DETAILED DESCRIPTION OF THE INVENTION

Suitable natural graphites are those with a high crystallinity, i.e.,with a crystalite size L_(c) greater than 100 nm and an ash content of amaximum of 5 percent. Suitable synthetic graphites also have a highcrystallinity L_(c) greater than 100 nm and a purity of 99.9 percent andmore. Preferably a synthetic graphite is used. The particle size (d₅₀)of the graphites used can vary in a range of 5 μm to 30 μm.

Clay minerals from the smectite class are used as an additionalnecessary component of the lubricant. The smectites essentially comprisesheet silicates and are distinguished because of their structure by ahigh cation exchange capability and by their swelling capability inwater (Ullmanns Encyklopadie der Techn. Chemie [Ullmann's Encyclopediaof Industrial Chemistry], 4th edition, VCH Weinheim, Vol. 23, pp.311ff.). From the smectite group, montmorillonites are preferably usedthat have a swelling capability (1 g of montmorillonite in distilledwater) of 3 to 50. Due to the above-mentioned cation exchangecapability, the montmorillonites can be "modified" with inorganic ororganic cations.

The above-mentioned clay minerals as an inorganic lubricant componentare distinguished by excellent binding properties and, moreover, havethe advantage that, in contrast with polymers or oils, they are notsubjected to pyrolysis. Moreover, in the lubricant, the above-mentionedclay minerals are substantially responsible for a surprisingly quickdrying time, in the range of 1 up to 5 seconds, of the lubricant film onthe mandrel. In other words, in view of the mentioned short time betweenapplication of the lubricant and the rolling operation with thelubricant according to the invention, that is, a maximum of 5 seconds, auniform and dry lubricant film on the mandrel can be produced evenbefore the mandrel is inserted in the ingot.

As another necessary component of the lubricant according to theinvention, a polysaccharide or a derivative of it is used as thickeningagent. This lubricant component has the object of assuring a constantviscosity of the lubricant dispersion over a wide temperature range aswell as to prevent the sedimentation of the solid particles in thedispersion. Biopolysaccharides, such as, xanthan gum, rhamsan gum, or analkyl cellulose derivative, such as, hydroxypropylmethyl cellulose, fromthe series of polysaccharides or their derivatives are suitably used.

To protect the thickening agent from a bacterial attack, suitably acommercially available biocide is added.

To obtain good film properties of the lubricant and to influence theviscosity of the lubricant, additionally a non-ionic surfactant issuitably used. (When a non-ionic surfactant is used, the amount is inthe range of about 0.001 to 5 percent by weight.)

Suitably used as non-ionic surfactants are those as described inUllmanns Encycl. der Techn. Chemie, VCH Weinheim, 4th Ed., Vol. 22, page489. Especially suitable representatives of this class are theoligomeric oxyethylates or the oxyethylates modified with oxypropylgroups (Ullmanns Encycl. der Techn. Chemie, loc. cit., pp. 489ff.).

The lubricant according to the invention is suitably used in the form ofan aqueous dispersion with a solid content of usually 20 to 40 percentby weight. But these range limits can be varied up or down based on theactual use situation. The production of the dispersion can take place incommercially available dispersion devices that make high shearing forcespossible.

The coating of the lubricant on the mandrel can take place e.g., by aspray ring placed between the cooling bath and the rolling mill, throughwhose center the mandrel rod is conveyed and is evenly provided withlubricant.

As a rule, the coating amount is controlled so that about 40 g/m² oflubricant (without water) is on the mandrel.

EXAMPLE 1

    ______________________________________                                        Formulation 1 (F1)                                                            synthetic graphite type T75 LONZA                                                                     86.8% by weight                                       modified montmorillonite with                                                                         11.2% by weight                                       a swelling capability of 10-50                                                methylhydroxypropyl cellulose                                                                         1.9% by weight                                                                100.0% by weight                                      dispersion of H.sub.2 O with solid content                                                            24.6% by weight                                       Formulation 2 (F2)                                                            synthetic graphite type T75 LONZA                                                                     78.2% by weight                                       montmorillonite with a  20.0% by weight                                       swelling capability of 9-14                                                   surfactant synperonic PE/F68                                                                          1.2% by weight                                        of the ICI Co.                                                                (oxyethylate modified oxypropyl groups)                                       xanthan gum             0.5% by weight                                        biocide                 0.1% by weight                                                                100.0% by weight                                      dispersion / H.sub.2 O with solid content                                                             30.0% by weight                                       Formulation 3 (F3)                                                            natural graphite with an                                                                              77.0% by weight                                       ash content of 4.5%                                                           montmorillonite with a  3.0% by weight                                        swelling capability of 10-30                                                  montmorillonite with a  18.0% by weight                                       swelling capability of 3-5                                                    surfactant synperonic PE/F68                                                                          1.2% by weight                                        of the ICI Co.                                                                (oxyethylate modified oxypropyl groups)                                       rhamsan gum             0.7% by weight                                        biocide                 0.1% by weight                                                                100.0% by weight                                      dispersion in H.sub.2 O with solid content of 30%                             by weight                                                                     Formulation 4 (F4)                                                            synthetic graphite type LONZA T75                                                                     90.9% by weight                                       montmorillonite type with a                                                                           7.0% by weight                                        swelling capability of 10-50                                                  surfactant synperonic PE/F68                                                                          1.2% by weight                                        of the ICI Co.                                                                (oxyethylate modified oxypropyl groups)                                       xanthan gum             0.7% by weight                                        biocide                 0.2% by weight                                                                100.0% by weight                                      dispersion in H.sub.2 O with solid content of 30%                             by weight                                                                     ______________________________________                                    

Comparison formulation 1, VF1 (according to German Patent No. 2450716)

20% of graphite

9.5% of vinylacetate copolymer

1% of polysaccharide

69.5% of water

Comparison formulation 2, VF2 (graphite-containing mineral oil)

35% of graphite

65% of mineral oil

Drying Test

The described dispersions were sprayed on a test mandrel at atemperature of 100° C. The drying time was measured.

                  TABLE                                                           ______________________________________                                        Formulation  Amount [g/m.sup.2 ]                                                                          Drying Time [s]                                   ______________________________________                                        F1           36.8 (150° C.)                                                                        5.0                                               F2           26.0           2.0                                               F3           40.0           2.0                                               F4           25.0           0.5-1                                             VF1          40             15                                                VF2          immersion process                                                                            no drying                                         ______________________________________                                    

What is claimed is:
 1. A process comprising coating a mandrel in theproduction process of seamless tubes, which has passed through thecooling bath after a rolling operation takes place and is being preparedfor the next rolling operation, with a mandrel lubricant, the mandrelbeing at a temperature of about 60° to 100° C., the mandrel lubricantcomprising:(a) 60 to 95 percent by weight of a member selected from thegroup consisting of natural graphite and synthetic graphite, the naturalgraphite having a crystal size L_(c) greater than 100 nm and an ashcontent of a maximum of 5 percent, and the synthetic graphite having acrystal size of L_(c) greater than 100 nm and a purity of 99.9 percentor higher; (b) 5 to 40 percent by weight of montmorillonite having aswelling capability in water of 3 to 50; (c) 0.2 to 2 percent by weightof at least one member selected from the group consisting ofpolysaccharide and one of its derivatives; (d) 0.001 to 5 percent byweight of a non-ionic surfactant; and (e) sufficient water to form saidaqueous dispersion, the total weight of (a) to (d) being 100 percent byweight.